Electron beam signal generators using parametric pump or similar amplifying section



1966 R. ADLER 30,473

ELECTRON BEAM SIGNAL GENERATORS USING PARAMETRIC PUMP OR SIMILAR AMPLIFYING SECTION Filed Dec. 18, 1961 kmuccnxw flamin o 2K wwom ucom INVENTOR.

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Roberi c/Qcizel United States Patent ELECTRON BEAM SIGNAL GENERATGRS USING PARAMETRIC PUlVIP 0R SIMILAR AMPLIFYING SECTION Robert Adler, Northfield, Ill., assignor to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed Dec. 18, 1961, Ser. No. 160,078 7 Claims. (Cl. 331-80) This invention pertains to electron-beam signal generators. More particularly, it relates to high frequency signal generators employing an aperiodic gain producing mechanism.

Travelling-wave tubes and similar devices designed for extremely high frequency operation are difficult to build from a practical standpoint because at such frequencies the interaction between the electron beam and external structure becomes inefficient. Because of this inefficiency, the gain obtainable at such frequencies with conventional devices is small.

In contrast with the interaction of travelling-wave tubes and the like, the mechanism of parametric amplification affords means which, at least in theory, permits the amplification of waves on an electron stream regardless of frequency. Parametric amplification of electron motion on a beam does not involve interaction with an external structure in the sense in which the conventional travellingwave tube principle is understood. Instead parametric amplification involves unilateral application of an externally applied field to the electron beam.

It is accordingly a general object of the present invention to provide a signal generator incorporating the electron beam parametric amplification mechanism in order to achieve efiiciency of operation.

It is another object of the present invention to provide an improved high frequency electron beam oscillator.

A further object of the present invention is to provide a new and improved high frequency signal generator which may be constructed of simple parts and basic assemblies in order to facilitate fabrication for operation at extremely high frequencies.

In accordance with one form of the present invention, a signal generator comprises means for projecting an electron beam along a predetermined path together with means for establishing resonance of the electrons in the beam at a predetermined frequency. An electron coupler is disposed adjacent -a first portion of the path and has a signal interaction frequency substantially higher than the electron resonance frequency. Field producing means adjacent a second portion of the path subjects the electrons to a periodic inhomogeneous field having a phase relationship with the electron motion to deliver energy to a component thereof in proportion to the component amplitude; this increases the signal level on the electrons. Means disposed adjacent a third portion of the path returns the increased energy level electrons adjacent the electron coupler and into interaction therewith. Finally, amplified signal energy is extracted from the beam.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:

FIGURE 1 is a schematic diagram of one embodiment of. a high-frequency oscillator; and

FIGURE 2 is a schematic diagram of an embodiment of the present invention.

The signal generator represented in FIGURE 1 for purposes of explaining the present invention includes an electron gun 10, an input coupler 11, a parametric expander 12, an output coupler 13, and a collector 14. In basic function, these elements operate in the manner described and claimed in the copending application of Glen Wade, Serial No. 747,764 filed July 10, 1958, and now abandoned in favor of its continuation application Serial No. 289,792 filed June 20, 1963, entitled Parametric Amplifier and assigned to the same assignee as the present invention. This particular form of electron beam parametric amplifier is premised upon basic principles disclosed in the copending application of Robert Adler, Serial No. 73 8,456 filed May 28, 1958, entitled Electronic Signal Amplifying Methods and Apparatus, and assigned to the same assignee as the present invention. Certain of the basic principles are also disclosed in an article by Robert Adler entitled Parametric Amplification of the Fast Electron Wave, appearing in the Proceedings of the IRE, volume 46, No. 6, June, 1958. The specific amplifying mechanism herein embodied is described and explained in detail in an article by Adler et al. entitled A Low-Noise Electron-Beam Parametric Amplifier appearing in the Proceedings of the IRE, volume 46. No. 10, for October, 1958.

Generally speaking, an electron beam is projected from gun 10 along a path 15 which extends through the first coupler, the expander, and the second coupler, and then terminates in the collector. The electron gun itself may be of any conventional type suitable for projecting a pencil-like electron beam.

During its passage through the various elements, the electron beam is subjected to means for establishing resonance of its electrons. As illustrated, this means takes the form of a magnetic field H oriented with its lines of flux along beam ath 15 and having a strength assigned to establish for the electrons cyclotron resonance at a frequency w in accordance with the well known cyclotron relationship. The establishment of cyclotron resonance creates a condition for the electrons under which a transverse force on the electrons causes them to orbit around the beam path. Because of the longitudinal drift velocity which was previously given them in the gun region, the electrons describe helical orb-its. Consequently, signal motion imparted to the electrons by interaction with coupler 11 causes the electrons to describe expanding helical orbits, the radius of which is representative of the signal energy amplitude and the periodicity of which corresponds to the cyclotron frequency.

The orbiting electrons, to which the input coupler has imparted electron signal motion, are then projected through quadrupole expander 12. As explained fully in the above mentioned Wade application and in the October 1958 IRE article, expander 12 subjects the orbiting electrons to a periodic inhomogeneous field having a phase relationship with the electron signal motion to deliver energy to a component thereof in linear proportion to the amplitude of that component. The quadrupole structure preferably takes the form of four electrodes symmetrically spaced circumferentially around the beam path and driven by an external source of pump signal energy, at a frequency u to develop the periodic inhomogeneous field. The effective pump signal frequency for optimum efficiency is approximately equal to twice the cyclotron resonance frequency established by the magnetic field.

Upon leaving quadrupole expander 12, the electron beam is caused to pass through output coupler 13 to which it gives up signal energy corresponding to its orbital electron motion. Subsequent to extraction of the amplified signal energy by coupler 13, the electrons are intercepted by collector 14 which is suitably connected to a source of positive potential indicated by the symbol B+.

Couplers 11 and 13 have an interaction frequency which is substantially higher than the cyclotron and pump frequencies, and the apparatus includes means for delivering energy from the amplified signal motion to the initial signal-coupling or motion-imparting means in a manner such that the delivered energy has a phase relationship with respect to the input-coupler signal energy proper to augment the electron signal motion on the beam. To this end, a band-pass filter 17 is coupled to deliver energy from output coupler 13 to input coupler 11. Filter 17 is assigned a pass-band selective of the signal frequency a to be amplified. The filter is coupled between the input and output couplers by impedance matching transformers 18. In practice at high frequencies, transformers 18" are in the form of waveguide devices and'the' like. While the output means may comprise a separate electron coupler interacting with the beam at any point therealong at the amplified signal frequency, it conveniently takes the form of a link to the output coupler and in the case illustrated constitutes an additional ouput port included on output transformer 18.

Electron couplers 11 and 13 may be of any suitable form to interact with the beam at a frequency different from, and in this case substantially higher than, the cyclotron frequency of the device. They may, for example, be constructed as a bifilar helix in which the pitch of the interleaved windings is assigned a value with respect to the orbital pitch of the electron motion to satisfy the necessary phase velocity relationships more fully described and explained in the co-pending application of Robert Adler entitled Electron Beam Amplifier and Apparatus therefor, Serial No. 119,931, filed June 27, 1961, and assigned to the same assignee as the present application. As illustrated in the present application, electron couplers 11 and 13 are of the interdigital or iterative kind described and claimed in the co-pending application of Robert Adler entitled Resistive Loading of Electron Beams on Adjacent Circuit Structures, Serial No. 34,961, filed June 9, 1960, and assigned to the same assignee as the present application. Because of the substantial difference between the signal frequency m and the cyclotron frequency w signal couplers 11 and 13 are constructed to develop signal waves having a finite phase velocity. This is consistent with the design of expander 12 for interaction at an infinite phase velocity and therefore permits use of the elemental quadrupole expander mentioned above.

Each of the couplers comprises a first series or fingers of conductive strips 20 positioned to one side of beam path 15 and extending transversely thereof and a like second series of strips 21 positioned symmetrically on the opposite side of the beam path. Alternate strips of series 20 are connected together and alternate members of series 21 are likewise connected together. Moreover, the two series are interconnected so that transformers 18 are connected across the structure with facing members as well as contiguous members of the two series 20 and 21 at opposite instantaneous polarities. Transformers 18 are designed to tune the couplers to exhibit sharp resonance at a desired signal interaction frequency.

In the transverse-mode type of electron discharge devices illustrated, the signal interaction may develop both a fast wave and a slow wave on the electron beam, and ;heir respective phase velocities may be expressed as folows:

where u =the electron velocity in the axial direction w =the radian cyclotron frequency w the radian frequency of the signal modulation.

When the cyclotron and signal frequencies are equal or close to one another in value, the fast and slow signal waves are sufliciently differentiated from one another in velocity that a coupler may :be easily constructed to respond selectively to one or the other. Specifically, for a case where the signal and cyclotron frequencies are the same, the phase velocity of the fast Wave is infinite, and a simple lumped coupler in the form of two deflector plates disposed symmetrically on opposite sides of the beam path provides effective coupling only to the fast wave signal. However, in the present instance the signal frequency is several times the cyclotron frequency as a result of which the absolute magnitudes of the phase velocities of the fast and slow waves approach one another, and it is therefore more diflicult to separate these two waves in a coupling structure on the basis of phase velocities. Nevertheless, this desirable result is obtainable with the interdigital coupler illustrated which may be dimensioned to achieve efficient coupling to the fast wave and substantially zero effective coupling to the slow wave. Additionally, dimensioning of the coupler .permits determination of the beam loading resistance in accordance with the following expression:

VD 2a 2 1r 2 7l'Ol/L 2 era 01 i fit) 1(5) l r 2 L E J where V =longitudinal accelerating voltage determining beam velocity.

a=spacing of each series 20, 21 from the beam axis.

l=overall dimension of the coupler along the beam axis.

a=half the spacing between contiguous members of the series.

L=distance between corresponding points of any two contiguous members of either series.

R =loading at the signal frequency.

The resistive and reactive components of impedance observed at the coupler terminals may be adjusted by varying beam current and beam velocity, respectively, and usually it is arranged that the coupler represents a predominantly resistive impedance of a predetermined value. Of course, if its terminal impedance is not suitable for direct connection to transformer 18,an impedance network of any well known construction may be interposed between the coupler terminals and the connected system component.

To have the electron coupler selective to the fast, as distinguished from the slow wave, it is dimensioned to achieve an integral number of full cycles phase difference of the slow relative to the fast wave over the entire length of the coupler. It will be apparent from Equations 1 and 2. that the absolute magnitude of the fast wave velocity is larger than that of the slow wave velocity. For optimum coupling in respect of the fast wave the following expression is to be satisfied:

At the same time, for slow wave cancellation as minimum coupling in respect of the slow wave the following condition is to be satisfied:

trons in the resonant beam is initiated merely in response to random electron motion corresponding to noise on the beam. The coupler interaction causes the electrons to follow an orbital cyclotron path and the radius of electron motion increases along the length of the coupler. Following passage through coupler 11, the electron beam is directed through quadrupole expander 12 in which the beam is parametrically expanded. As a result, the energy of the electron signal motion at the desired signal frequency is increased, representing gain or amplification of the signal motion initially produced in coupler 11. This increased-energy signal motion is then caused to interact in coupler 13 to deliver energy by way of output transformer 18 to the load circuit. Additionally, a portion of the amplified signal energy present on coupler 13 is transmitted by way of band-pass filter 17 back to electron coupler 11 where it is applied to the coupler with a phase relationship which augments the signal motion initially on the beam. This regenerative or oscillatory feedback together with the amplification obtained in quadrupole expander 12 causes a continued reinforcement of the energy carried by the electron beam and consequently results in the production of high level signal energy as derived by the load.

In accordance with one aspect of the present invention, the electron beam upon which amplified Wave signal energy has been developed and is carried is caused to again traverse the same signal coupler utilized for the initial signal modulation of the beam. In the embodiment illustrated in FIGURE 2, a single electron coupler 25 cooperates with a parametric expander 26 and an electron mirror 24 to generate high frequency signal energy. Electron coupler 25 in this instance is an interdigital coupler constructed in the manner described with reference to couplers 11 and 13. The only connection necessary to coupler 25 is that represented by a load, in this case matched to the coupler by a wave-guide transformer 28 coupled across the coupler elements in the same manner as that described above with respect to transformer 18.

Quadrupole expander 26 may take the same form as expander 12 and is in this instance of the infinite phase velocity type illustrated to include four electrodes 27 circumferentially spaced around a beam path. The pairs of oppositely facing ones of electrodes 27 are connected across a transformer 29 in turn coupled to a pump source 30.

The electron beam projected from gun 10 through electron coupler 25 and expander 26 is caused to retrace its path through the expander by electron mirror 24. As i illustrated, the electron mirror comprises an apertured repeller 32 followed by focusing electrode 33. Electrodes 32 and 33 are connected to suitable potential sources illustrated by the symbols B3 and B4, respectively; the magnitudes of the potential sources are adjusted relative to the DC. potential on the cathode of electron gun 10, so as to cause the electron beam to be reversed in direction in the conventional manner.

In order that the returning electron beam will be directed so as to avoid impingement upon the cathode of electron gun 10, a beam deflector preferably is included along the beam path between coupler 25 and expander 26. This conveniently takes the form of a pair of deflector plates 35 across which a potential source 36 is connected to divert the beam transversely of the beam path. Since it is desired for optimum efiiciency that the beam pass centrally through expander 26, electron gun 10 preferably is olfset slightly from the longitudinal axis of deflector plates 35. The fiield across deflectors 35 has a strength suflicient to deflect the beam to a position along the central beam path 15 in its passage from deflector plates 35 through the expander and to the electron mirror. On its return trip from electron mirror 27 to coupler 25, the electron beam is again caused to deviate by deflector 35, but this time in a direction away from the central beam path opposite that in which the initial beam path portion in coupler 25 was spaced. As a result, the returning beam upon its passage through coupler 25 is caused to impinge upon a suitable intercepting portion of the electron gun. Of course, a separate annular electrode may be interposed between the electron gun and coupler 25 for this purpose.

In operation of the apparatus illustrated in FIGURE 2, coupler 25 initially interacts with the electrons of the beam to develop signal motion at an interaction frequency selected to have a value several times that of the electron resonance or cyclotron frequency established by magnetic field H. Upon subsequent passage through expander 26, the energy from pump source 30 subjects the orbiting electrons to a gain producing field in the manner above described with respect to expander 12. After reversal of direction under the influence of electron mirror 24, the beam again traverses expander 26 and the electron signal motion is subjected to further amplification in the same manner as upon the initial passage. Finally, the beam is caused to pass through deflectors 35 and to re-enter coupler 25 where the amplified signal motion interacts with the coupler elements and delivers amplified signal energy through transformer 28 to the load in a manner like that explained above with respect to coupler 13 of FIGURE 1. At the same time the returned beam reinforces the initial modulation imparted to the beam by the coupler thereby sustaining oscillation with a resultant build up of a high level of signal energy for supply to the load. Of particular note is that the FIGURE 2 embodiment takes advantage of the unusual property of an interdigital coupler to permit finite phase velocity interaction with waves of a given length traveling in either direction on the electron beam.

In an alternative to that shown in FIGURE 2, the electron beam passes through a finite phase velocity expander. The other sections of the device are the same as shown in FIGURE 2 except that in this instance the cyclotron frequency established by magnetic field H is different from that realized in the apparatus of FIGURE 2. The field strength is selected so that the ratio of signal to cyclotron frequencies is such that the expander supports a field propagating at a finite velocity. That is, the propagation constant of the pump field must be such that the electron pattern representing the signal modulation imposed by coupler 25 will effectively see a field having a proper phase velocity for parametric amplification. It can be shown that the sum of the propagation constants of the desired signal wave, the pump signal wave and the idler signal wave always must be equal to zero; the idler signal necessarily is developed at a frequency equal to the difference between the pump and signal frequencies. In accordance with these relationships, the finite pump phase velocity is in this instance obtained by constructing the expander of four helical transmission lines circumferentially spaced around the beam path in the manner of electrodes 27 of expander 26 of FIGURE 2. The diagonally opposite pairs of the transmission lines are connected across a pump source delivering the pump signal energy preferably at a frequency which appears to the moving electron to be twice that of the cyclotron resonance established by magnetic field H. Such an expander may also be used in the device of FIGURE 1. Moreover, other finite phase velocity expanders may be utilized in both FIGURES 1 and 2; for example, lumped electrodes similar to those shown in FIGURE 2 of expander 26 may be employed but skewed around the beam path so that with respect to the electron stream the pump field has a finite phase velocity. Such finite phase velocity expanders are described and claimed in the aforementioned copending application Serial No. 747,764 filed July 10, 1958 by Glen Wade.

Since the development of noise energy usually is unimportant in signal generators, the pump signal frequency may be lower than that of the developed signal even though this necessarily means the concurrent development of a slow-wave idler signal which may contribute noise to the output signal. While this may be accomplished with the finite phase velocity expanders just discussed, a different expander is especially advantageous. It is composed of four electrodes similar to those of expander 26 but twisted around the beam path in the direction of the electron orbits to define four corresponding interleaved helicoidal loci of the same pitch as that of and co-directional with the helical electron orbit path. The pump energy is supplied from a unidirectional source, such as a battery, which is connected across adjacent ones of the electrodes.

In operation, this expander develops a symmetrical quadrupole field which appears to rotate around the beam path in a direction therealong. Since the pitch of the electrodes corresponds to that of the electron orbits, the expander field simulates a symmetrical non-homogeneous pumping field rotating in synchronism with the electrons and it expands the radius of their orbital motion in the same manner as described with respect to expander 26 of FIGURE 2. The arrangement therefore permits the generation of extremely high-frequency signal energy while requiring energization only with direct current. An expander like that just discussed, however, imparts gain to the electrons orbiting in but one direction. Expander 50 is described and claimed in the co-pending application of Glen Wade, Serial No. 840,336 filed September 16, 1958 and assigned to the same assignee as the present application.

-An expander which imparts gain for electrons orbiting in either direction, which may be termed a DC. pumped interdigital quadrupole, develops a static quadrupole field by means of a plurality of electrodes arranged in groups of four. The electrodes of each group are symmetrically spaced around the beam path and the groups are spaced therealong. The electrodes are energized with direct current as indicated so that each is of a polarity opposite its neighbor in both the circumferential and axial directions. In operation with such an expander substituted for expander 26 in the device of FIGURE 2, an electron of appropriate phase gains rotational energy as it loses drift energy; in this respect the epander operates in a manner somewhat similar to that of a conventional traveling wave tube. The proper synchronism. condition requires that where v is the electron drift velocity, w is the cyclotron frequency and L is the periodic spacing of the quadrupole fields. In consequence, the electrons experience an apparent pump field having a frequency of 2 and exponential gain is achieved in the same manner as with expander 26 of FIGURE 2 but in both directions of electron travel. A typical embodiment of this kind of expander, in a different combination, is described and illustrated in a publication entitled A Transversed-Field Traveling- Wave Tube by E. I. Gordon in the Proceeding of the IRE, vol. 48, No. 6, June 1960, at page 1158.

Having once gained an appreciation of the concepts underlying the present invention, other structural arrangements readily suggest themselves. Any bi-directional expander is most useful in a device like that of FIGURE 2; such an expander may also be used in apparatus as in FIGURE 1. On the other hand, unidirectionally acting expanders may be used in any of the embodiments but preferably in those in which the beam traverses the expander in one direction. The electron stream may be caused to reciprocate several times, traversing an expander at each passage. Alternatively, the beam may travel a generally circular path passing through the expander in only one direction. By enforcing the travel of the electron stream in a closed path so as to repeatedly tnavense an expander, regenerative signal development may be sustained even without an external signal coupler; of course, known techniques such as spaced gaps, selective electrode lengths, and the like may be employed to suppress undesired modes of operation and select only that which is desired.

The various embodiments described all have an advantage of enabling the generation of extremely high frequency signals. Because of the high level of gain obtainable in the apparatus discussed, satisfactory operation may be obtained even though rather poor efficiencies are obtained in the couplers. In all forms the apparatus is comparatively simple of construction and any mechanical tolerances involved are not at all severe.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. Signal generating apparatus comprising:

means for projecting an electron beam along a predetermined path;

means for establishing resonance of electrons in said beam at a predetermined frequency;

an electron coupled disposed adjacent a first portion of said path having a signal interaction frequency substantially higher than said predetermined frequency;

field producing means disposed adjacent a second portion of said path for subjecting said electrons to a periodic inhomogeneous field having a phase relationship with motion of said electrons to deliver energy to a component of said motion in proportion to the amplitude of said component, increasing the signal energy level of electrons to which signal energy is imparted by said coupler;

means disposed adjacent a third portion of said path for causing said increased-energy-level electrons to return adjacent to said electron coupler and into interaction therewith;

and means for extracting amplified signal energy from said beam.

2. Signal generating apparatus comprising:

means for projecting an electron beam along a predetermined path;

means for establishing resonance of electrons in said beam at a predetermined frequency;

an electron coupler disposed adjacent a first portion of said path having a signal interaction frequency substantially higher than said predetermined frequency;

field producing means disposed adjacent a second portion of said path for subjecting said electrons to a periodic inhomogeneous field having a phase relationship with motion of said electrons to deliver energy to a component of said motion in proportion to the amplitude of said component, increasing the signal energy level of electrons to which signal energy is imparted by said coupler;

means disposed adjacent a third portion of said path for causing said increased-energy-level electrons to return adjacent to said electron coupler and into interaction therewith;

and a load coupled to said electron coupler for extracting amplified signal energy from said beam at said signal interaction frequency.

3. Signal generating apparatus comprising:

means for projecting an electron beam along a predetermined path;

coupling means for imparting to electrons in said beam motion representative of signal energy at a predetermined frequency;

means for establishing resonance of said electrons at a frequency substantially lower than said predetermined frequency;

field producing means f-or subjecting said electrons to a periodic inhomogeneous field having a phase relationship with said motion to deliver energy to a component thereof in proportion to the amplitude of said component and thereby amplify said signal motion;

means disposed on said path beyond said field producing means for causing said beam to return through said field producing means and into signal interaction with said coupling means at said predetermined frequency;

and means for deriving amplified signal energy from said beam.

4. Signal generating apparatus comprising:

means for projecting an electron beam along a predetermined path;

coupling means for imparting to electrons in said beam motion representative of signal energy at a predetermined frequency;

means for establishing cyclotron resonance of said electrons at a frequency substantially lower than said predetermined frequency;

field producing means for subjecting said electrons to a periodic time-variable inhomogeneous field having a phase relationship with said motion to deliver energy to a component thereof in proportion to the amplitude of said component and thereby amplify said signal motion;

means disposed on said path beyond said field producing means for causing said beam to return into signal interaction with said coupling means at said predetermined frequency;

and means for deriving amplified signal energy from said beam.

5. Signal generating apparatus comprising:

means for projecting an electron beam along a first path;

means for establishing resonance of electrons in said beam at a predetermined frequency;

an electron coupler disposed adjacent a first portion of said path having a signal interaction frequency substantially higher than said predetermined frequency;

field producing means disposed adjacent a second portion of said path for subjecting said electrons to a periodic inhomogeneous field having a phase relationship with motion of said electrons to deliver energy to a component of that motion in proportion to the amplitude of said component, increasing the signal energy level of electrons to which signal energy is imparted by said coupler;

means disposed adjacent a third portion of said path for causing said increased-energy-level electrons to return into interaction with said coupler along a second path portion within said coupler spaced from the portion of said first path therein;

and means for extracting amplified signal energy from said beam.

6. Signal generating apparatus comprising:

means for projecting an electron beam along a predetermined path;

coupling means for imparting to electrons in said beam motion representative of signal energy at a predetermined frequency;

means for establishing cyclotron resonance of said electrons at a frequency substantially lower than said predetermined frequency;

field producing means for subjecting said electrons in both directions of travel along said path, to a periodic inhomogeneous field having a phase relationship with said motion to deliver energy to a component thereof in proportion to the amplitude of said component and thereby amplify said signal motion;

means disposed on said path beyond said field producing means for causing said beam to return through said field producing means and into signal interaction with said coupling means at said predetermined frequency;

and means for deriving amplified signal energy from said beam.

7. Signal generating apparatus comprising:

means for projecting an electron beam in one direction along a predetermined path;

coupling means for imparting to electrons in said beam motion representative of signal energy at a predetermined frequency;

means for establishing cyclotron resonance of said electrons at a frequency substantially lower than said predetermined frequency;

field producing means for subjecting said electrons to a periodic inhomogeneous field having a phase relationship with said motion to deliver energy to a component thereof in proportion to the amplitude of said component and thereby amplify said signal motion;

means disposed on said path beyond said field producing means for causing said beam to return in the other direction along said path into signal interaction with said coupling means at said predetermined frequency;

beam-deflection means disposed along said path between said coupling means and said field producing means for laterally deflecting said beam to one side when traveling in said one direction and to the other side when traveling in the other direction;

means for intercepting said returning heam after said interaction with said coupling means;

and means for deriving amplified signal energy from said beam.

References Cited by the Examiner UNITED STATES PATENTS 7/1952 Field 3153 5/1958 McBride 315-5.52 8/1963 Ashkin 330-4.7

FOREIGN PATENTS 3/1960 France. 4/1960 Russia.

ROY LAKE, Primary Examiner. JOHN HUCKERT, Examiner. 

1. SIGNAL GENERATING APPARATUS COMPRISING: MEANS FOR PROJECTING AN ELECTRON BEAM ALONG A PREDETERMINED PATH; MEANS FOR ESTABLISHING RESONANCE OF ELECTRONS IN SAID BEAM AT A PREDETERMINED FREQUENCY; AN ELECTRON COUPLED DISPOSED ADJACENT A FIRST PORTION OF SAID PATH HAVING A SIGNAL INTERACTION FREQUENCY SUBSTANTIALLY HIGHER THAN SAID PREDETERMINED FREQUENCY; FIELD PRODUCING MEANS DISPOSED ADJACENT A SECOND PORTION OF SAID PATH FOR SUBJECTING SAID ELECTRONS TO A PERIODIC INHOMOGENEOUS FIELD HAVING A PHASE RELATIONSHIP WITH MOTION OF SAID ELECTRONS TO DELIVER ENERGY TO A COMPONENT OF SAID MOTION IN PROPORTION 